As technology which is used by an X-ray computed tomography apparatus to automatically set a tube current, auto exposure control (AEC) is known, which automatically determines a tube current in accordance with an X-ray absorption index value, such as a water-equivalent thickness. When the step & shoot process is executed in the AEC to acquire data while moving a top plate intermittently, a tube current value is set for an imaging region regarding each stop position of the top plate. An average value of the X-ray absorption index values of the pixels included in each imaging region is set as an X-ray absorption index value of the imaging region. In the step & shoot process, if there is a rapid shape change, as in the head, the tube current undergoes a step-like change in the adjacent imaging region, causing a difference in an image Standard Deviation (SD). This is attributable to the fact that each image region involves a large number of structures since a recent X-ray computed tomography apparatus employs a larger number of X-ray detectors and the data on a wide range can be acquired at a time.